The processes of this patent disclosure are concerned with removal of cyanide species such as free cyanide, complex heavy metal cyanide, etc. from metal-containing industrial waste water effluents in general and from precious metal-containing mill tailing slurries in particular. Cyanides are of course widely used in recovering various precious and/or base metals from their ores. For example, gold and silver are often recovered by treating their ores with a cyanide solution in order to dissolve and separate the gold and silver values from the ore's gangue. Thereafter, the resulting gold and silver cyanide leach solution is further processed (e.g., by the addition of zinc, carbon or resin) to recover the precious metals. During the course of such recovery operations, cyanide ions are known to: (1) form complexes with a host of heavy metals (e.g., complexes having the general formula [Me(CN).sub.x.sup.v- ]), (2) remain in free form, and (3) take the form of certain related ionic species such as thiocyanates (SCN.sup.-) and/or cyanates (CNO.sup.-) Therefore, applicants' use of general terms such as "cyanide" and "cyanide species" are intended to include any and all chemical species which contain a cyanide (CN.sup.-) group.
The methods employed to deal with cyanide-containing mill tailings have included: (1) oxidation by chemical and/or electrochemical means, (2) ion exchange and (3) so-called AVR (acidification-volatilization-regeneration) processes. All of these methods are operative, but, to some degree or other, each is hampered by various practical or economic disadvantages. Not the least of these is the high reagent costs normally associated with treating the huge quantities of aqueous effluents generated by precious metal mining operations. By way of example, chemical oxidation of those cyanide species contained in waste water effluents can be carried out with costly, strong oxidants such as chlorine gas, hypochlorite, ozone, peroxide, peroxysulfates and/or oxygen, usually with the aid of select catalysts. Of these processes, alkaline chlorination is perhaps the most commonly used oxidation process employed in the precious metal mining industry. Other potential oxidation agents such as peroxides and peroxysulfates are likewise very expensive reagents, especially when they are used in the context of the dilute cyanide solutions normally produced by mining operations. On the other hand, ozonation treatments of such effluents are particularly disadvantaged by their especially high capital and energy costs.
Many such oxidation processes also require costly catalysts for their efficient operation. By way of example, pressure oxidation processes require large amounts of nickel metal or activated carbon catalysts in order to effectively destroy the cyanide content of the subject effluent. They also require expensive pressure vessels. Electrolytical oxidation of such aqueous cyanide solutions also has been tried, but such processes involve the use of sophisticated electrolytic cells and, once again, very expensive capital equipment. Ion exchange methods have also been used in recovering metal values and cyanide, but their use has been hampered by high capital and operating costs and complicated by other chemical process problems. Moreover, virtually every one of the above noted HCN removal and/or recovery methods involves rather sophisticated chemical technologies. Consequently, careful control, of a kind not customarily available at remote mine sites, is a very real practical limitation to many of these processes.
In response to all of the above-noted problems various processes have been suggested wherein cyanide species contained in waste water effluents can be decomposed and/or recovered through the use of less expensive reagents and/or equipment. For example, U.S. Pat. No. 4,622,149 teaches reduction of cyanide, arsenic and antimony levels in an aqueous stream by adding ferric ion (in water-soluble form) to a cyanide containing effluent while treating it with gaseous SO.sub.2 and oxygen in the presence of a soluble copper catalyst. The overall effect of this process is to decompose free and complex metal cyanide species and to precipitate arsenic and antimony in association with a ferric hydroxide precipitate.
U.S. Pat. No. 4,615,873 teaches a process for reducing cyanide levels in certain ferro-cyanide solutions. The precipitation is accomplished with the aid of certain metal reagents, other than copper, such as zinc, which are employed either prior to or simultaneously with, treatment of the ferro-cyanide solution with sulfur dioxide and air in the presence of a soluble copper catalyst.
U.S. Pat. No. 4,537,686 teaches removal of cyanide from aqueous solutions by a process which essentially comprises exposing the solution to a mixture of sulfur dioxide and air (or oxygen) in the presence of a water-soluble copper catalyst. The copper catalyst catalyzes the removal of free cyanide, complex heavy metal cyanide cyanate and/or thiosulfates. After the cyanide species are removed, other species are removed by continued treatment with sulfur dioxide and air (or oxygen) in the presence of another metal ion catalyst such as those of nickel, cobalt or manganese.
It should also be noted that those processes which are based upon injection of SO.sub.2 gas into mill tailings, or which are based upon the use of sulfurous acid reagents, have yet another drawback. They must accept, and pay the economic consequences of, the fact that limestone is present in many ores and that limestone is reactive with SO.sub.2. That is to say that any limestone present in a cyanide-containing aqueous or slurry effluent may have to be neutralized before the SO.sub.2 can begin to neutralize the cyanide. Moreover, SO.sub.2 is known to produce certain sulfur species which could possibly interfere with certain other precious metal recovery techniques, e.g., flotation and Merrill-Crowe processes. It also should be pointed out that even if limestone were not present in an ore, many of the above noted processes which introduce SO.sub.2 gas into mill tailings in order to remove cyanide also employ limestone as the reagent of choice in achieving and/or controlling the pH of the processes. For example, the pH of the process taught by U.S. Pat. No. 4,537,686 is maintained between 5 and 6.5 by the use of limestone. Consequently, as in the case of an ore which contains limestone, any SO.sub.2 gas which is introduced to recover cyanide will first react with such limestone reagent before any appreciable amounts of cyanide can be recovered.
Thus, any process which: (1) is capable of effectively removing cyanides from waste waters, (2) uses lower cost reagents, (3) does not need limestone to adjust pH levels, (4) is not in fact consumed by a limestone component of an ore and (5) can be carried out with relatively simple equipment having low capital and energy costs would be welcome in any industry having a need to treat cyanide-containing effluents and especially in those remote sites where precious metal mining operations often take place.